1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device and a display device using a thin film transistor in at least a pixel portion.
2. Description of the Related Art
In recent years, a technique that is used to form thin film transistors using semiconductor thin films (with thicknesses of from several nanometers to several hundreds of nanometers, approximately) formed over substrates having an insulating surface has been attracting attention. Thin film transistors are widely applied to electronic devices such as ICs or electro-optical devices, and development of thin film transistors which are to be used as switching elements especially in image display devices is being pushed.
As a switching element in an image display device, a thin film transistor using an amorphous semiconductor film, a thin film transistor using a polycrystalline semiconductor film, or the like is used. As a method for forming a polycrystalline semiconductor film, a technique is known in which a pulsed excimer laser beam is shaped into a linear laser beam by an optical system and an amorphous silicon film is scanned and irradiated with the linear beam to be crystallized.
Furthermore, as a switching element in an image display device, a thin film transistor using a microcrystalline semiconductor film is used (see Patent Document 1: U.S. Pat. No. 5,591,987 and Patent Document 2: Japanese Published Patent Application No. H4-242724).
Moreover, as a method for manufacturing a thin film transistor using a microcrystalline semiconductor film, a method is known in which after an amorphous silicon film is formed on a gate insulating film, a metal film is formed on a surface of the amorphous silicon film, and the metal film is irradiated with a diode laser so that the amorphous silicon film is modified into a microcrystalline silicon film (For example, see Non-Patent Document 1: Toshiaki Arai et al., SID 07 DIGEST, 2007, pp. 1370-1373). According to this method, the metal film formed over the amorphous silicon film is a film for converting optical energy of the diode laser into thermal energy, and should be removed later in order to complete the thin film transistor.
However, there are problems in that a process becomes complicated when steps of forming and removing a metal film for crystallization of a semiconductor film are performed as in Non-Patent Document 1, and accordingly yield is reduced and cost is increased.